Chemical vapor deposition for epitaxial growth

ABSTRACT

An epitaxial layer of a group IIIa - Va element compound having practicable electron concentration and mobility such as those of GaAs, of less than 1 X 1016/cm3 at room temperature and more than 6,000 cm2/V-sec., respectively is obtained by reacting triethylaluminum which contains at most 4 ppm of silicon with a gallium halide or an indium halide to produce triethylgallium or triethylindium and contacting thus obtained triethylgallium or triethylindium alone, a mixture of them or a mixture of them and an alkyl compound of aluminum with a group Va element, a hydride of said element or an alkyl compound of said element.

United States Patent [191 Ichiki et a1.

[ CHEMICAL VAPOR DEPOSITION FOR EPITAXIAL GROWTH [75] Inventors: Eiichi Ichiki; Kazuo Iida; Masato Ogura, all of Niihama; Yasuo Seki;

Sizuo Asanabe, both of Kawasaki, all of Japan [73] Assignees: Sumitamo Chemical Company,

Limited, Osaka; Nippon Electric Company, Limited, Tokyo, both of, Japan 22 Filed: Mar. 13, 1974 21 Appl. No.: 450,682

[30] Foreign Application Priority Data Mar. 15, 1973 Japan 48-30664 [52] U.S. Cl 148/175, 148/1.5, 148/174 [51] Int. Cl. H011 7/36 [58] Field of Search 148/175, 174, 1.5; 260/429 R, 448 A; 357/3; 423/126, 240, 245, 495

[56] References Cited UNITED STATES PATENTS 3,312,570 4/1967 Ruehrwein 148/175 Feb. 18, 1975 1/1970 Conrad et a1. 148/175 10/1973 Chicotka et a1. 148/175 [57] ABSTRACT An epitaxial layer of a group 1110 Va element compound having practicable electron concentration and mobility such as those of GaAs, of less than 1 X 10 "/cm at room temperature and more than 6,000 cm /V-sec., respectively is obtained by reacting triethylaluminum which contains at most 4 ppm of silicon with a gallium halide or an indium halide to produce triethylgallium or triethylindium and contacting thus obtained triethylgallium or triethylindium alone, a mixture of them or a mixture of them and an alkyl compound of aluminum with a group Va element, a hydride of said element or an alkyl compound of said element.

13 Claims, No Drawings CHEMICAL VAPOR DEPOSITION FOR EPITAXIAL GROWTH The present invention relates to a method for vapor phase growth of an epitaxial layer of a compound of group llla Va elements (in the mendelejeffs periodic table) using an alkyl compound of group llla element as a source for the group llla element. More particularly, it concerns with a method for vapor phase growth of an epitaxial layer of a compound of group llla Va elements on a surface of a substrate which comprises reacting triethylaluminum which is prepared by specific synthesis methods and of which the silicon content is 4 ppm or lower (in accordance with the analysis method specified hereinafter) as a source for group Illa element with a halogenated gallium or indium to produce triethylgallium or triethylindium and then mixing, in vapor phase, thus obtained triethylgallium or triethylindium alone, a mixture thereof or a mixture of said compound and an alkyl compound of aluminum with a group Va element, a hydride of this element or an alkyl compound of this element.

The group llla Va element compound semiconductors such as GaP, GaAs, GaAs P Ga A] AS, lnP, etc. are extremely useful as materials for Gunn diode, superhigh frequency devices and electro-luminescent devices and demands therefor have recently been markedly increased.

Today, for vapor phase growth of an epitaxial layer of group llla Va element compound semiconductors, the following methods have been known; (1) the thermal decomposition method which comprises thermally decomposing an alkyl compound of a group llla element and a group Va element or a volatile compound thereof and (2) the halogen transport method which comprises reacting a chloride of a group llla element and a group Va element or a volatile compound thereof. At present, industrially effected methods are the halogen transport methods. The reason therefor is, as is clear from Journal of Electrochemical Society Vol. 116, No. 12, page l,729 (1969), that electron concentration at room temperature of an epitaxial layer formed from trimethylgallium and arsine is at most 1 X Hi /em and mobility thereof is as high as 5,000 cm /V-sec, both of which do not reach the practical level required for, e.g., Gunn diode, namely, electron concentration of less than 1 X IO /cm and mobility of more than 6,000 cm /V-sec. According to the halogen transport method, however, an epitaxial layer having an electron concentration of about 1 X l0 /cm at room temperature and a mobility of more than 6,000 cm lV-sec can be produced, but there are many problems in production thereof. That is, according to the halogen transport method, it is necessary that a reaction zone and a deposition zone are provided and the method is carried out under such condition as a slow gradient of temperature of 7 12C/cm being provided near substrate. Therefore, the area suitable for growth is limited to extremely narrow area. Furthermore, usually only several 30 pieces can be produced at one time and productivity is low. Furthermore, it is difficult to produce mixed crystals of two or more such as (Ga, Al)As, (Ga, ln)P, etc. On the other hand, according to the thermal decomposition method, growth of epitaxial layer can be attained by heating only the substrate, area of growth zone can be optionally selected, productivity can be increased and moreover, since group llla Va element compound is deposited by thermal decomposition of an alkyl compound of Group llla element, mixed crystals such as (Ga, Al)P, (Ga, ln)P, etc. can easily be produced. Thus, the drawbacks encountered in the halogen transport method as mentioned above can be overcome. However, up to date, epitaxial layers having practically usable electron carrier concentration and mobility for electron devices have not been able to be produced by the thermal decomposition method and the thermal decomposition method has not yet been industrialized.

The inventors, in an attempt to produce an epitaxial layer having practically usable electron concentration and mobility by thermal decomposition method, synthesized alkylgallium by reaction of (a) Grignards reagent and a gallium halide, (b) dialkylmercury and metallic gallium and (c) dialkylzinc and a gallium halide which are generally known as methods for synthesis of an alkyl compound of a group lIIa element and they purifled thus obtained alkylgallium to find that this alkylgallium inevitably contains Mg, Hg and Zn. Therefore, when the alkylgallium synthesized by said methods was used as a starting material, grown cyrstals contains, as background impurities, said group II elements in an amount similar to that in the starting material. Thus, it was impossible to reduce electron concentration of the crystals at room temperature to less than 1 X lO /cm Furthermore, practically usable epitaxial layer could not also be obtained even when an alkylgallium obtained by other methods for synthesis of an alkyl compound of group llla element, namely, from an alkylaluminum and a gallium halide was sufficiently purified with distillation. The same thing can be applied to in case of alkylindium. In view of such circumstances, the inventors have made an intensive research as to the reason why epitaxial layers produced using alkylgallium or alkylindium prepared with use of an alkylaluminum as an alkylating agent cannot be practically utilized and they found that the epitaxial layer produced from said starting material contains silicon. The inventors supposed that this incorporation of silicon into the epitaxial layer was caused by the alkylaluminum used for synthesis of alkylgallium or allkylindium. However, since no method for determination of a slight amount of silicon compound in alkylaluminum has been proposed, they firstly established a method for determination of amount of silicon compound in alkylaluminum and then made a detailed research. As the result, they found that silicon compound is incorporated due to aluminum which is a starting material for synthesis of alkylaluminum and is present as various organosilicon compounds in a wide range of boiling point and that some of said silicon compounds are difficultly separatable from alkylgallium or alkylindium by distillation, but the silicon compounds can easily be separated from alkylaluminum compounds by distillation. However, when an epitaxial layer is produced using alkylgallium or alkylindium synthesized from alkylaluminum compound from which silicon compound has been removed and gallium halide or indium halide, in case of the alkyl being methyl a large amount of carbides are incorporated into epitaxial layer to cause reduction of crystalgroup Va element. Based on these discoveries, the present invention was accomplished.

The object of the present invention is to provide a method for vapor phase growth of a high purity epitaxial layer having practicable electron concentration and mobility, such as those GaAs, namely, less than 1 X l/cm at room temperature and higher than 6,000 cm /V-sec, respectively from an alkyl compound of group Illa element.

The above object of the present invention to produce an epitaxial layer of group Illa Va element compound on a surface of a substrate by mixing an alkyl com pound of a group Illa element and a group Va element or a compound thereof in vapor phase and contacting the reaction product with the substrate is attained as follows: I Triethylaluminum which is synthesized by (i) reaction of triethylaluminum, diethylaluminum hydride or a mixture thereof, aluminum, hydrogen and ethylene, (ii) reaction of an alkylaluminum compound, aluminum and hydrogen or aditionally an olefin to produce an alkylaluminum compound, which is then subjected to alkyl substitution with ethylene or (iii) reaction or reducing ethylaluminum sesquihalide obtained by reaction of aluminum and an ethyl halide and in which silicon content is made 4 ppm or lower is reacted with 11 gallium halide or indium halide to produce triethylgallium or triethylindium and Ill thus obtained triethylgallium or triethylindium alone, a mixture thereof or a mixture of said compound with an alkyl compound of aluminum is mixed with IV a group Va element or a hydride or an alkyl compound of said element in vapor phase to form an epitaxial layer of group Illa Va element compound on a surface of a substrate.

According to the present invention, triethylgallium or triethylindium is synthesized using triethylaluminum of which the silicon contant is at most 4 ppm determined in accordance with the method of analysis as specified hereinafter. The triethylaluminum of which the silicon content is at most 4 ppm is synthesized by either one of the following methods: (i) triethylaluminum, diethylaluminum hydride or a mixture thereof, aluminum, hydrogen and ethylene are reacted, (ii) alkylaluminum compound, aluminum and hydrogen are reacted or these are further reacted with an olefin to obtain an alkylaluminum compound, which is then subjected to alkyl substitution with ethylene and (iii) aluminum and ethyl halide are reacted to obtain ethylaluminum sesquihalide, which is then reduced. Said reactions (i) and (ii) may be accomplished by the methods disclosed in Japanese Patent Publications No. 57l0/57, No. 927/58 and US. Pat. No. 2,835,689 and the reaction (iii) also may be accomplished by the known reaction conditions.

As said alkylaluminum compounds, for example, trialkylaluminum or dialkylaluminum hydride having alkyl group of 3 20 carbon atoms such as tripropylaluminum, triisobutylaluminium, tri-2- ethylhexylaluminum, trioctylaluminum or hydrides thereof may be used. As the ethyl halides, ethyl chloride, ethyl bromide and ethyl iodide may be used.

The silicon compound which has become a problem in the present invention results from a slight amount of silicon contained in aluminum which is a starting material for synthesis of triethylaluminum and which is extracted as an organo silicone compound during synthesis of triethylaluminum and is incorporated therein. The commercially available triethylaluminum usually contains more than l0 ppm (in terms of silicon) of silicon compound determined by the analysis method of the present invention. The form of the silicon compound present in the alkylaluminum compound is not known and it is difficult to remove said silicon compound from triethylgallium or triethylindium synthesized using triethylaluminum containing the silicon compound. Therefore, silicon content of triethylaluminum used for synthesis of triethylgallium or triethylindium must be reduced to 4 ppm or lower before being used for reaction. Triethylaluminum whose silicon content is 4 ppm or lower may be obtained by purification of triethylaluminum synthesized by the method (i), alkylaluminum compound or triethylaluminum synthesized by the method (ii) or ethylaluminumsesquihalide or triethylaluminum synthesized by the method (iii) by the known purification means such as rectification, recrystallization, etc. It is preferred to conduct filtration and the like prior to the purification to reduce load in purification. It is essential in the present invention that the alkylaluminum compound used for synthesis of triethylgallium or triethylindium is triethylaluminum and the reason therefor will be given hereinafter.

The high purity triethylaluminum which is synthe sized by said specific methods and of which the silicon content is made 4 ppm or lower is then reacted with gallium halide or indium halide by the known method to synthesis triethylgallium or triethylindium. As the gallium halides, gallium chloride, gallium bromide or gallium iodide is used and as the indium halides, indium chloride, indium bromide or indium iodide is used. As the gallium halides and the indium halides, commercially available high purity products may be used or these may be purified by sublimation. Furthermore, they may be synthesized by blowing a halogen into molten gallium or indium metal and then purified by sublimation.

The reaction between triethylaluminum and gallium halide or indium halide proceeds in accordance with the following equation:

MX 3Al(C H M(C I-I 3(C H AlX (wherein M is gallium or indium and X is a halogen atom). In synthesis of triethylgallium or triethylindium, at least 3 mols, usually 3 4 mols of triethylaluminum is used per 1 mol of gallium halide or indium halide.

The synthesis reaction may be generally carried out at a temperature of 0 150C and under a pressure of normal pressure, but if necessary, under reduced pressure or higher pressure. Furthermore, the reaction may also be carried out in the presence of a solvent having a boiling point of 30 C such as petroleum ether, pentane, hexane, etc.

Thus synthesized triethylgallium or triethylindium is vacuum distilled or is maintained at 0 C. for about 10 minutes 2 hours with addition of potassium chloride, sodium fluoride or a mixture thereof before or after the vacuum distillation to fix by-product diethylaluminum halide as a complex and thereafter is rectified to obtain purified triethylgallium or triethylindium. It is essential that alkyl group of alklylgallium of alkylindium is ethyl and in case alkyl group is methyl, carbides in a large amount are incorporated into the obtained epitaxial layer to cause reduction in crystallinity of the group Illa Va element compound and in case alkyl group is propyl or higher alkyls, vapor pressure is low and such alkylgallium and alkylindium are not suitable as starting materials for growth of crystal. As mentioned before, many methods for synthesis of triethylgallium or triethylindium have been known other than the method of the present invention, but in accordance with these known methods, group II element is inevitably incorporated and practically usable product cannot be obtained.

The triethylgallium or triethylindium obtained from triethylaluminum whose silicon content is 4 ppm or lower as mentioned above alone or in a form of a mixture thereof or in admixture with an alkylaluminum compound is mixed with a group Va element, a hydride thereof or an alkyl compound thereof in vapor phase by known method to form an epitaxial layer on a surface of a substrate.

As the group Va element, arsenic or phosphorus may be used. As the hydride or alkyl compound of the group Va element, phosphine; alkylphosphines such as triethylphosphine, diethylphosphine, monoethylphosphine, tripropylphosphine, etc.; arsine; alkylarsines such as triethylarsine, diethylarsine, monoethyl arsine, monopropylarsine, etc.; or ammonia may be used. Use of phosphorus trichloride and arsenic trichloride is not preferred as the compound of group Va element because in this case, growth of crystal is difficult. As the alkylaluminum compound to be used in admixture with triethylgallium or triethylindium, triethylaluminum, diethylaluminum hydride, tripropylaluminum, dipropylaluminum hydride, dibutylaluminum hydride, etc. may be used and preferably silicon content in these compounds is less than 1 ppm per aluminum atom.

The growth of crystal may be carried out by the known method and known apparatus. Gas of alkyl compound of group llla element is introduced into crystal growing zone at a rate of (0.05 X 10' mol/min and group Va element, or hydride or alkyl compound thereof is introduced into the crystal growing zone at a rate of (0.05 50) X 10' mol/min. Of course, hydrogen, argon or a mixture thereof may be introduced as a carrier together with the alkyl compound of group Illa element and group Va element or a compound thereof. As well known, the same crystal as the growing layer, silicon, sapphire, spine], etc. may be used as the substrate. Growth of crystal is generally effected at a temperature of 500 1,300C.

ln practising the method of the present invention, a reactor made of the usual refractories such as quartz, porcelain, carborundum, graphite, etc. may be used. The gaseous reagents are introduced in individual stream or in a mixture stream into the reactor. Heating of substrate in the reactor is generally carried out by induction heating so that the group Illa Va element compound is deposited on only the surface of the substrate.

The method of the present invention has been explained with reference to growth of a high purity crystal, but donor or acceptor may be added to the growing layer by the conventional method at the time of growth of crystal.

Semiconductors having the epitaxial layer produced by the method of the present invention can sufficiently be used for Gunn diode, superhigh frequency devices and electro-luminescent devices.

According to the method of the present invention detailedly explained hereinbefore, it becomes possible to produce an epitaxial layer having practically usable electric characteristics from alkyl compound of group llla element from which an epitaxial layer having practically usable electric characteristics has not been conventionally able to be produced. The industrial significance thereof is extremely great. Furthermore, productivity is increased to more than 2 times that of the conventional halogen transport method. Moreover, according to the conventional halogen transport method, production of mixed crystals such as Ga,ln .,P, 0a,. AI As, etc. and growth of epitaxial layer different from substrate are difficult. On the other hand, according to the present invention, it is easy to produce mixed crystals and it is also easy to form an epitaxial layer different from substrate.

Quantitative analysis of silicon in triethylaluminum was carried out in accordance with the following analysis.

Sample Test:

120 ml of water and 50 ml of hexane were charged in a quartz flask of 300 ml and 10 g of triethylaluminum was gradually added dropwise thereto to cause hydrolysis. The produced aluminum hydroxide was vacuum dried at C for 10 hours. 3 g of thus obtained dried aluminum hydroxide was placed on a platinum dish of 50 ml and 6 of sodium carbonate and 2 g of boric acid were added thereto. They were well stirred and the dish was covered with a platinum cover. This was preheated on a sand bath for 40 minutes and then was kept in an electric furnace at 1,000C for 20 minutes. Thereafter, the content was left for cooling and water was added thereto and this was heated to dissolve the content in water. The solution was transferred to a polyethylene beaker of 200 ml. Said platinum dish was washed with 1 cc of 8N nitric acid with warm and the washing solution was transferred to said beaker. Then, 20 ml of 8N nitric acid was further added to the content of the beaker. The beaker was covered with a polyethylene watch glass and the content was warmed to completely dissolve the precipitate. The solution was cooled and then transferred into a polyethylene measuring flask to attain predetermined volume. This was employed as a sample solution. 30 ml of this sample solution was taken and charged in a polyethylene beaker of ml. The pH of this solution was adjusted to 0.8 with ammonia water (if necessary, nitric acid is used) and was transferred into a polyethylene measuring flask of 50 ml. The solution was added 2.5 ml of an ammonium molybdate solution prepared by dissolving 10 g of ammonium molybdate in 100 ml of water and they were stirred and allowed to stand for 5 minutes. To the solu tion was further added 2.5 ml of 3N nitric acid and 2.5 ml of a sulfonic acid reduced solution (a mixture of solution A: solution prepared by dissolving 7 g of sodium sulfite and L5 g of l-amino-2-naphthol-4-sulfonic acid in 100 ml of water and solution B: solution prepared by dissolving 90 g of sodium bisulfite) and water was added thereto to obtain a predetermined volume. A part of this solution was taken in a cell of spectrophotometer and absorbance of the solution to a light of 825 m in wavelength was measured using water as a comparison.

Blank Test:

A part of solution prepared in the same manner as mentioned above except that the dried aluminum hydroxide was not added was taken in a cell of the spectrophotometer and absorbance of this solution to a light "of 825 m in wavelength was measured.

Calibration Curve:

2 Gelof silicic acid re-percipitated from water glass with hydrochloric acid (1 l) was washed with warm water until no chlorine ion was detected and then was dried at 100 i 5C. A portion of this silicic acid was dissolved in aqueous sodium hydroxide solution in a polyethylene vessel and this was diluted with suitable amount of water to prepare a silicic acid standard solution. A portion of this solution was taken in a cell ofthe spectrophotometer and absorbance of the solution to a Silicon content in triethylaluminum (p.p.m.)

A: Amount of silicon in the sample solution (g) B: Amount of silicon in the blank test solution (g) C: Amount of the sample solution used (m) W: Amount of aluminum hydroxide (g) F: Conversion factor of triethylaluminum and dried aluminum hydroxide In case of the above method, F 0.6. The following reagents were used in the above analysis.

Water: This was prepared by distilling an ion exchanged water in a quartz still to obtain water having a specific resistance of 1,500,000 wcm (30C). l-lexane: This was prepared by treating a guaranteed grade hexane with sulfuric acid and then distilling it. Anhydrous sodium carbonate, boric acid, ammonia water (charged in a polyethylenebottle) and nitric acid (charged in a polyethylene bottle): These were commercially available high purity agents. Ammonium molybdate, sodium sulfite and l-amino-2-naphthol-4-sulfonic acid: Commercially available guaranteed reagents ln carrying out the crystal growth of group Illa Va element compound using an alkyl compound ofa group llla element in accordance with the present invention, it is natural that purity of group Va element or a hydride of this element is also very important, but in the Examples hereinafter, raw materials commercially available as those for semiconductors were used.

Thepresent invention will be illustrated in the following Examples, but they do not limit the scope thereof in. any way.

EXAMPLE 1 30 atm., 70 75C) were alternately charged and 7 they were reacted to obtain triethylaluminum. Thusob tained triethylaluminum was filtered and distilled and furthermore rectified through a packed column having theoretical plates to obtain triethylaluminum containing 0.5 ppm of silicon.

On the other hand, metallic gallium was placed in a transparent quartz reactor and was molten. Then, chlorine was blown into the melt to produce gallium chloride. Thus obtained gallium chloride was purified with sublimation.

450 parts of triethylaluminum obtained above was charged in a borosilicate glass (Pyrex glass) reactor. A gallium chloride-petroleum ether solution prepared by dissolving 207 parts of gallium chloride obtained above in 650 parts of petroleum ether was added dropwise to said triethylaluminum in one hour with stirring and reaction was effected for further one hour. After completion of the reaction, the petroleum ether was distilled out under normal pressure and then triethylgallium was separated from diethylaluminum chloride and triethylaluminum by fractional distillation under reduced pressure. To the separated triethylgallium were added 20 parts of potassium chloride and 18 parts of sodium fluoride and they were stirred at C for 1 hour to fix triethylaluminum and diethylaluminum chloride present in a slight amount as a complex. Thereafter, they were rectified through a borosilicate glass (Pyrex glass) of 15 mm X 1,000 mm packed with a Pyrex glass spiral rings to obtain 162 parts of high purity triethylgallium. ii. 800 Parts of triisobutylaluminum, 250 parts of aluminum and 1,800 parts of isobutylene were charged in an autoclave and hydrogen was charged therein until 200 atm reached. This was heated to C to produce triisobutylaluminum. Thus obtained triisobutylaluminum was allowed to contact with ethylene at 100C to cause alkyl exchange reaction to produce triethylaluminum. This triethylaluminum was filtered and distilled and furthermore was rectified through a packed column having 70 theoretical plates to obtain triethylaluminum containing 2.4 ppm of silicon. Then, triethylgallium was prepared in the same manner as in (i). iii. 43 Parts of aluminum and 2 parts of ethylaluminum sesquibromide were dissolved in methylcyclohexane. 250 Parts of ethylbromide was added dropwise thereto and they were reacted at 50 100C. Furthermore, 80 parts of K-Na (2 1) alloy was added and reaction was carried out at 100 180C to prepare triethylaluminum. Thus obtained triethylaluminum was filtered and distilled and furthermore was rectified through a packed column having 70 theoretical plates to obtain triethylaluminum containing 1.5 ppmof silicon. Triethylgallium was prepared in the same manner as in (i). iv. Metallic indium was placed in a transparent quartz reactor and chlorine was blown thereinto at 200C to produce indium chloride. This indium chloride was purified with sublimation. Triethylindium was produced from said indium chloride and triethylaluminum containing 0.5 ppm of silicon obtained in (i) in the same manner as the method of production of triethylgallium in (i). v. The triethylaluminum produced in (i) was subjected to simple distillation to obtain triethylaluminum containing 10 ppm of silicon and triethylgallium was produced in the same manner as in EXAMPLE 2 This Example illustrates epitaxial growth of GaAs using triethylgallium and arsine as starting materials. a.

As a substrate, semiinsulating GaAs crystal of 100) in which Cr was doped was heated to 650C. When triethylgallium produced in (i) of Example 1 and arsine were flowed over the substrate at 0.5 X 10 mol/min and 1.5 X 10 mot/min respectively and hydrogen was also flowed through this system at 2.5 /min, epitaxial growth took place and growth rate of GaAs was 20 /hr. Electrical properties of the grown layer measured by Hall measurement are as follows: Electron concentration at room temperature was 1 X l"/cm and mobility was 9,000 cm /V-sec. b. The above procedure (a) was repeated except that triethylgallium prepared in (ii) of Example I was used to cause epitaxial growth. As the result, electron carrier concentration at room temperature was 3 X /cm and mobility was 8,000 cm /V-sec. c. The above procedure (a) was repeated except that triethylgallium prepared in (iii) of Example 1 was used to cause epitaxial growth. Electron concentration at room temperature was 6 X IO /cm and mobility was 8,500 cm /V-sec. d. The above procedure 9a) was repeated except that triethylarsine was used in place of arsine. The electrical properties of the layer were nearly the same as those of (a). e. For comparison, the above procedure (a) was repeated except that triethylgallium prepared in (v) of Example 1 was used. As the result, electron concentration of the layer at room temperature was 1.3 X 10""lcm and mobility was 4,100 cm /V-sec.

From the above results, it is clear that silicon content in triethylaluminum used for producing triethylgallium must be at most 4 ppm.

EXAMPLE 3 This Example illustrates epitaxial growth of GaP with use of triethylgallium and phosphine or phosphorus. a. As a substrate for crystal growth, a semi-insulating GaP crystal of (l( 0) direct io n in whichghromium was doped in a high concentration was heated to 810C. When triethylgallium obtained in (i) of Example 1 and phosphine were flowed over said substrate at 0.5 X 10 mol/min and 2 X 10 mol/min and hydrogen was flowed at 2.5 l/min through this system, rate of the epitaxial growth of GaP was [.L/hl. Electrical properties of the grown layer were measured in the same manner as in Example 2 to obtain an electron concentration at room temperature of 3 X IO /cm and a mobility of 180 cm /V-sec. b. When above procedure (a) was repeated except that in place of phosphine, yellow phosphorus was heated and supplied to the reaction system at 3 X 10" mol/min, growth of GaP crystal was possible and the epitaxial layer had an electron concentration of 1.3 X l0"/cm and a mobility of 165 cm /V-sec.

EXAMPLE 4 This Example illustrates epitaxial growth of GaAlAs and application thereof to lightemitting diode.

As a substrate, (100) GaAs crystal doped with Te and having an electron concentration of l X l0 /cm was heated to 800C and H Te diluted to 100 ppm was added to the system in such a manner that electron concentration of the grown layer was 1 X l0"/cm and at the same time, triethylgallium prepared in (i) of Example l, arsine and H were flowed through the system at 5 X 10' mol/min, 2 X 10* mol/min and 2.5 l/min, respectively to carry out epitaxial growth of GaAs on the GaAs substrate for 30 minutes. Then, triethylaluminum containing 0.5 ppm of silicon obtained in (i) of Example 1 was gradually added to the system. After lapse of 1 hour, flow velocity of triethylaluminum fed to the system was made.2 X 10' mol/min and at this concentration, growth was continued for additional 1 hour.

Thus obtained epitaxial layer contained aluminum in a concentration of x 0.39 in Ga Al As. In this crystal, P-N junction was formed by Zn diffusion method using the general sealed tube method and then a diode was produced. Luminance of red emission at 20 mA was 200 fL. This value is substantially the same as that of the commercially available GaAsP luminous diode. In case of adding no H Te as an impurity to the system, electron concentration of the epitaxial layer wax 3 X l0 /cm and sufficiently high purity crystal can be obtained by this method.

EXAMPLE 5 This Example illustrates growth of lnGaP crystal and characteristics of a luminous diodle to which said crystal was applied.

As a substrate, GaP crystal doped with l X lO /cm of Te was heated to 830C. While phospine was flowed through the system at a flow rate of 3 X 10 mol/min, 100 ppm of H Te was also flowed therethrough so that the electron concentration of the grown layer reached 7 X IO /cm and then triethylgallium obtained in (i) of Example 1 was supplied at a flow rate of 3 X 10 mol/min using hydrogen as a carrier to effect growth of GaP crystal for 30 minutes. The total amount of hydrogen which flowed through the system at this time was 2 l/min. Then, triethylindium obtained in (iv) of Example 1 was gradually added to this system so that flow rate of triethylindium after lapse of l hour reached 1 X l0- moi/min and the growth of crystal was continued for additional 1 hour under this condition. Thereafter, the growth was discontinued. The resultant crystal contained gallium in a concentration of x 0.7 in In Ga P. A diode produced by Zn diffusion with a sealed tube showed yellowish green emission of a luminance of fL at 50 mA. In case of adding no H Te as an impurity, electron concentration of the grown layer was 8 X 10 /cm What is claimed is:

1. In a method for producing an epitaxial layer of a group IIIa Va element compound on surface of a substrate which comprises reacting an alkyl compound of a group IIIa element with a group Va element or a volatile compound thereof in vapor phase and contacting the reaction product with the substrate, the improvement which comprises reacting triethylaluminum which contains at most 4 ppm of silicon with a gallium halilde or an indium halide to produce triethylgallium or triethylindium and contacting thus obtained triethyl gallium or triethylindium alone, a mixture of them or a mixture of them and an alkyl compound of aluminum with a group Va element, a hydride of said element or an alkyl compound of said element.

2. A method according to claim 1, wherein said triethylaluminum is prepared by a reaction of triethylaluminum, diethylaluminum hydride or a mixture thereof, aluminum, hydrogen and ethylene.

3. A method according to claim 11, wherein said triethylaluminum is prepared by reacting an alkylaluminum compound, aluminum and hydrogen or reacting an alkylaluminum compound, aluminum, hydrogen and 1 1 an olefin and subjecting thus obtained alkylaluminum to alkyl exchange reaction with ethylene.

4. A method according to claim 1, wherein said triethylaluminum is prepared by reducing an ethylaluminum sesquihalide obtained by reaction of aluminum and'an ethyl halide.

5. A method according to claim 1, wherein the gallium halide is selected from gallium chloride, gallium bromide and gallium iodide.

6. A method according to claim 1, wherein the indium halide is selected from indium chloride, indium bromide and indium iodide.

7. A method according to claim 1, wherein at least 3 mols of triethylaluminum is used per 1 mol of gallium halide or indium halide in the reaction of triethylaluminum and gallium halide or indium halide.

8. A method according to claim 1, wherein the reaction of triethylaluminum and gallium halide or indium halide is carried out at a temperature of 150C under normal pressure.

9. A method according to claim 1, wherein the group Va element is selected from arsenic and phosphorus.

10. A method according to claim 1, wherein the hydride or alkyl compound of the group Va element is selected from phosphine, triethylphosphine, diethylphosphine, monoethylphosphine, tripropylphosphine, arsine, triethylarsine, diethylarsine, monoethylarsine, monopropylarsine and ammonia.

11. A method according to claim 1, wherein the alkyl compound of aluminum mixed with triethylgallium or triethylindium is selected from triethyl aluminum, diethylaluminum hydride, tripropylaluminum, dipropylaluminum hydride and dibutylaluminum hydride.

12. A method according to claim 1, wherein the growth of crystal is carried out at 500 1,300C.

13. An epitaxial layer having an electron concentration at room temperature of less than 1 X IO /cm which is obtained by the method defined in claim 1. 

1. IN A METHOD FOR PRODUCING AN EPITAXAL LAYER OF A GROUP IIIA - VA ELEMENT COMPOUND ON SURFACE OF A SUBSTRATE WHICH COMPRISES REACTING AN ALKYL COMPOUND OF A GROUP IIIA ELEMENT WITH A GROUP VA ELEMENT OR A VOLATILE COMPOUND THEREOF IN VAPOR PHASE AND CONTACTING THE REACTION PRODUCT WITH THE SUBSTRATE, THE IMPROVEMENT WHICH COMPRISES REACTING TRIETHYLALUMINUM WHICH CONTAINS AT MOST 4 PPM OF SILICON WITH A GALLIUM HALIDE OR AN INDIUM HALIDE TO PRODUCE TRIETHYLGALLIUM OR TRIETHYLINDIUM AND CONTACTING THUS OBTAINED TRIETHYGALLIUM OR TRIETHYLINDIUM ALONE, A MIXTURE OF THEM OR A MIXTURE OF THEM AND AN ALKYL COMPOUND OF ALUMINUM WITH A GROUP VA ELEMENT, A HYDRIDE OF SAID ELEMENT OR AN ALKYL COMPOUND OF SAID ELEMENT.
 2. A method according to claim 1, wherein said triethylaluminum is prepared by a reaction of triethylaluminum, diethylaluminum hydride or a mixture thereof, aluminum, hydrogen and ethylene.
 3. A method according to claim 1, wherein said triethylaluminum is prepared by reacting an alkylaluminum compound, aluminum and hydrogen or reacting an alkylaluminum compound, aluminum, hydrogen and an olefin and subjecting thus obtained alkylaluminum to alkyl exchange reaction with ethylene.
 4. A method according to claim 1, wherein said triethylaluminum is prepared by reducing an ethylaluminum sesquihalide obtained by reaction of aluminum and an ethyl halide.
 5. A method according to claim 1, wherein the gallium halide is selected from gallium chloride, gallium bromide and gallium iodide.
 6. A method according to claim 1, wherein the indium halide is selected from indium chloride, indium bromide and indium iodide.
 7. A method according to claim 1, wherein at least 3 mols of triethylaluminum is used per 1 mol of gallium halide or indium halide in the reaction of triethylaluminum and gallium halide or indium halide.
 8. A method according to claim 1, wherein the reaction of triethylaluminum and gallium halide or indium halide is carried out at a temperature of 0* - 150*C under normal pressure.
 9. A method according to claim 1, wherein the group Va element is selected from arsenic and phosphorus.
 10. A method according to claim 1, wherein the hydride or alkyl compound of the group Va element is selected from phosphine, triethylphosphine, diethylphosphine, monoethylphosphine, tripropylphosphine, arsine, triethylarsine, diethylarsine, monoethylarsine, monopropylarsine and ammonia.
 11. A method according to claim 1, wherein the alkyl compound of aluminum mixed with triethylgallium or triethylindium is selected from triethyl aluminum, diethylaluminum hydride, tripropylaluminum, dipropylaluminum hydride and dibutylaluminum hydride.
 12. A method according to claim 1, wherein the growth of crystal is carried out at 500* - 1,300*C.
 13. An epitaxial layer having an electron concentration at room temperature of less than 1 X 1016/cm3 which is obtained by the method defined in claim
 1. 